1. Field of the Invention
The present invention relates to a slip and to a process for producing an oxidation- and corrosion-resistant diffusion layer.
2. Discussion of Background Information
In metallic components which are exposed to high temperatures, for example components of turbomachines in the form of stationary gas turbines or aircraft engines, the metallic surfaces have to be protected from oxidative and/or corrosive attack. Many different layer systems are known for this purpose.
Among other things, diffusion layers in which chemical elements diffuse into the metallic surface to be protected and/or deposit on the surface in order to give an increased concentration of the corresponding element on the surface to be protected are used. The element which has diffused in then provides, together with the alloy constituents of the base material, appropriate properties which allow the component to be operated at the desired high temperatures.
Thus, for example, providing aluminum diffusion layers on high-temperature materials such as cobalt- or nickel-based alloys is known, with the increased concentration of aluminum in the surface region of the corresponding alloys leading to a slowly growing aluminum oxide layer being formed in the case of high-temperature oxidative attack so as to protect the material from further damaging oxidative attack.
An appropriate aluminum diffusion layer can be produced, inter alia, by application of a slip comprising aluminum-containing powder particles which provide the aluminum for the diffusion process; after drying and/or hardening of the slip on the surface to be treated, the aluminum diffuses from the dried slip layer into the material during subsequent diffusion heat treatment. The application of such a slip by painting, dipping or spraying is very simple, and such a process is therefore of industrial interest for producing an aluminum diffusion layer. In particular, components can also be repaired in a simple way by the simple application.
Known slips for carrying out an aluminum diffusion process comprise aluminum-containing powder particles together with a binder which essentially provides the liquid phase for forming the slip. However, such a slip has to have such a constitution that the aluminum-containing powder is oxidized very little by the binder, so that the subsequent diffusion process is not made difficult by the presence of aluminum oxide. Accordingly, provision of additional chemical compounds such as chromates or dichromates and phosphates in the aqueous and acidic binders is known. Furthermore, silicon powder can be mixed in in order to assist the diffusion process.
Chromates in particular have been used in the past to increase not only the oxidation resistance but also the corrosion resistance of the metallic component which has been treated correspondingly and in addition also to inhibit the oxidation of the metallic aluminum in the slip. However, chromium(VI) compounds are extremely toxic and hazardous to health, and so attempts are increasingly being made to replace these components in the slip compositions. Examples are described in WO 2010/134917 A1, WO 2010/134918 A1, WO 2013/149609 A1, WO 2010/022698 A1 or DE 198 07 636 C1, the entire disclosures of which are incorporated by reference herein.
However, there continues to be a need for slips and processes for producing oxidation- and corrosion-resistant diffusion layers which make do without toxic Cr(VI) compounds which are hazardous to health while at the same time providing a balanced oxidation protection and corrosion protection for metallic components at high temperatures under aggressive environmental conditions.
In view of the foregoing, it would be desirable to have available a slip and a process for producing diffusion layers for protection of metallic components against oxidation and corrosion at high temperatures and under aggressive environmental conditions, in which the slip can be produced and applied simply and the process can be carried out simply and reliably without posing a hazard to the health of the user.